Clean up FrameLoader stop functions

cancelAndClear() seems unnecessary.
stopForUserCancel() appears close enough to stopAllLoaders()
stop() seems unnecessary.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/22439010

git-svn-id: svn://svn.chromium.org/blink/trunk@156114 bbb929c8-8fbe-4397-9dbb-9b2b20218538